The invention relates to optical components particularly with respect to controlling the polarization of optical radiation.
In addition to having color, light waves have the attribute of polarization. Light is a superposition of two orthogonal states of polarization. We can consider one of these states to be an oscillation of the wave up and down along a vertical direction. The other state will be an oscillation left and right along a horizontal direction. If the two states are oscillating in phase, then, at a given point in space, the electric field vector of the light wave traces, in time, a segment along a straight line. The light is said to be linearly polarized. When the two states are oscillating partially out of phase, then the electric vector traces an ellipse. In particular, when they are out of phase by 90 degrees and the oscillation amplitudes are equal, then the electric vector traces a circle, and the light is circularly polarized. Light is circularly polarized, right-handed or left-handed, when the two states are out of phase by +90 or xe2x88x9290 degrees respectively. The properties of polarized light are described in detail in D. Clarke et al., Polarized Light and Optical Measurement, (Pergamon Press, Oxford, 1971).
Scientists understand that when linearly polarized light is incident on the surface of a material, the light which is reflected has a component which is circularly polarized. Such sources of linearly polarized radiation are pervasive in the visible and infrared wavelengths. Some examples are the polarization found in light from the daytime blue sky, in the thermal emission from the ocean surface, and in scattered light underwater. It is surprising then, that imaging sensors which are sensitive to circular polarization have not been developed for object detection and recognition. Recent measurements reported in K. P. Bishop et al., xe2x80x9cMulti-spectral polarimeter imaging in the visible to near IR,xe2x80x9d in Targets and Backgrounds: Characterization and Representation V, W. R. Watkins, D. Clement, and W. R. Reynolds, Eds, Proceedings of SPIE Vol. 3699, 49-57(1999), suggest that, in the visible and near infrared wavelength range, as much as 5 percent of ambient light is circularly polarized. Another advantage of using circular polarization images is that the sign and magnitude of the circular polarization can potentially be used to reveal the spatial orientation, material, and surface roughness of the object""s surface.
The generation by reflection of circularly polarized light is enhanced when the surface is smooth or the surface material is electrically conductive. These characteristics are more common in man-made surfaces than in natural surfaces. A sensor imaging circular polarization would be able to detect man-made objects in a background almost free of clutter. Such a sensor could have applications in automobiles to alert drivers of the presence of other vehicles, especially at night, in fog, or in rain. Military applications include the detection of vehicles placed among trees and shrubs.
In ambient scenes, there is potentially as much variety and information in polarization images as there is in color images. However, a practical and reliable method of imaging circular polarization has not been developed.
A method to detect circular polarization must distinguish between right-handed and left-handed polarized light. Most photodetectors are insensitive to polarization. So a filter must be placed in front of the detector which is able to transmit only right-handed or only left-handed polarized light. In practice, such a filter is a combination of a quarter-wave retarder and a linear polarizer.
As light travels through the retarder, the phase of the horizontal state of oscillation is shifted by 90 degrees relative to the vertical state of oscillation. Consider the case when right-handed circularly polarized light is incident on the retarder. The retarder tranforms the light into a linearly polarization state with oscillations along a direction rotated 45 degrees from horizontal.
The light then enters the linear polarizer. If the linear polarizer is oriented to transmit light oscillating along the direction rotated 45 degrees from horizontal, then the light will be incident on the photodetector. However, if the light originally had left-handed polarization, the light exiting the retarder would be linearly polarized along the direction rotated xe2x88x9245 degrees from horizontal and would not be transmitted by the linear polarizer. Hence, the combination of the quarter-wave retarder, linear polarizer, and photodetector will only respond to light containing right-handed circular polarization.
In contrast, if the linear polarizer is rotated so that it transmits light with oscillations along the direction rotated xe2x88x9245 degrees from horizontal, then the combination will respond only to left-handed circular polarized light. Similarly, the linear polarizer can remain fixed and the retarder rotated. At certain retarder orientations the combination will respond only to right-handed polarized light, and at certain other orientations only to left-handed polarized light. In a circular polarization sensor, the mechanism for rotating the linear polarizer or retarder introduces weight and cost and makes the sensor less reliable. In addition, the frequency at which the images can be updated, i.e. the frame rate, is limited by the rotation rate.
The simplest retarder is a plate, referred to, in the art, as a waveplate or phase plate, made of a birefringent material. Birefringent materials have a fast axis and a slow axis. Light waves, with oscillations along the direction of the fast axis, propagate with higher velocity than light waves, with oscillations along the slow axis. Because of this velocity difference, as light waves traverse a birefringent material, their two states of oscillations can be shifted in their relative phase. The phase shift as the light exits the waveplate is specified by choosing the thickness of the birefringent material.
Linear polarizers suitable for use in imaging sensors are know in the art. However, quarter-wave retarders with suitable characteristics have not been developed. The technology of retarders, are reviewed in J. M. Bennett et al., xe2x80x9cPolarization,xe2x80x9d in Handbook of Optics, W. G. Driscoll, Editor (McGraw-Hill, New York, 1978).
For the purpose of imaging circular polarization of light in ambient scenes, a retarder should have the following characteristics.
First, retarders should have achromatic response. Ambient light contains a range of wavelengths. However, quarter-wave retarders in the art are able to transform light with a phase shift of 90 degrees in only a very narrow range of wavelengths. For use in imaging circular polarization, a quarter-wave retarder, is needed which is achromatic over a wavelength range matching the wavelength range of the photodetector.
An infrared retarder, which in the art is considered to be achromatic, is described in U.S. Pat. No. 4,961,634 to Chipman et al. (1990). However, measurements that show this device is only approximately achromatic are reported in Sornsin et al. xe2x80x9cAlignment and calibration of an infrared achromatic retarder using FTIR Mueller matrix spectropolarimetry,xe2x80x9d in Polarization: Measurement, Analysis, and Remote Sensing, D. H. Goldstein et al., Eds, Proceedings of SPIE Vol. 3121, 28-34 (1997). These measurements show that over the wavelength range from 3 to 14 micrometers, the phase shift varies in a range from 74 to 98 degrees.
The device of Chipman et al. uses a combinations of two bulk crystals, cadmium sulfide (CdS) and cadmium selenide (CdSe). We use the term, bulk crystal, to refer to a macroscopic crystal as distinguished from a material formed as a thin film of microscopic thickness using methods related to the fabrication of integrated circuits.
An achromatic retarder which is a combination of two waveplates is described in M. G. Destriau et al., xe2x80x9cRxc3xa9alisation d""un quart d""onde quasi achromatique par juxaposition de deux lames cristallines de mxc3xaame nature,xe2x80x9d in J. Phys. Radium 10(2), 53-55 (1949). In order to convert circularly polarized light to linearly polarized light, light is first transmitted through a quarter-wave waveplate and then is transmitted through a half-wave waveplate. The two waveplates are made of the same birefringent material. However their fast axes are rotated relative to each other. By having a fixed angular displacement of the fast axes, the achromatic characteristics of the first waveplate is compensated by the second waveplate.
Destriau et al. explain the operation of their retarder by referring to Stokes parameters and the Poincarxc3xa9 sphere. Stokes parameters and the Poincarxc3xa9 sphere are described in detail in D. Clarke et al. A beam of incoherent radiation emitted or reflected from an object""s surface can be completely described at a given wavelength by the four Stokes parameters, (I, Q, U, V). The first Stokes parameter I is a measure of the total intensity of radiation. The second parameter Q measures the amount of linear polarization in the horizontal direction. The third parameter U measures the amount of linear polarization at 45 degrees from the horizontal. The fourth parameter V is a measure of the circular polarization. In the art, the Stokes parameters are often normalized by dividing the parameters by I. Then Q, U, and V are restricted to values in the range xe2x88x921 to 1.
FIG. 1 shows a three-dimensional plot in an orthogonal coordinate system with axes Q, U, and V. In this coordinate system, the Poincarxc3xa9 sphere 16 is a spherical surface of unit radius centered at the origin. Every possible state of polarized light is represented by a point on the sphere. In particular, the north pole of the sphere represents right-handed circular polarization. Points on the equator of the sphere represent linear polarization along directions at various angles from horizontal. A quarter-wave waveplate maps a point on the sphere to a point defined by a 90 degree rotation about an axis through the origin. The axis and direction of rotation is determined by the orientation of the fast axis of the birefringent material of the waveplate.
Referring to FIG. 1, in the device of Destriau et al., the quarter-wave waveplate transforms the right-handed circularly polarized component V=1 to linearly polarized component A. Similarly a half-wave waveplate maps a point on the sphere to a point defined by a 180 degree rotation about an axis through the origin. The half-wave waveplate of Destriau et al. then rotates the direction of linear polarization to change component A to horizontally linear polarized component U=1.
The retarder made from the combination of a quarter-wave waveplate and half-wave waveplate is achromatic. For example, if, at a different wavelength, the difference between the fast and slow velocities in the birefringent material is slightly larger by a factor m greater than 1, then, on the Poincare sphere 16, the component V will be transformed by the quarter-wave waveplate through a path m times longer to Axe2x80x2. In the half-wave waveplate, the path will also be m.times longer, so Axe2x80x2 will be transformed to a point close to U=1.
Destriau et al. show measurements made with waveplates formed from mica, which is a bulk crystal. The method of Destriau et al. was used in P. Hariharan et al., xe2x80x9cVariable achromatic polarization rotators,xe2x80x9d Opt. Eng. 36(9) 2563-2566 (1997). P. Hariharan et al. studied an achromatic polarization rotator using waveplates made of a bulk crystal, quartz.
In the above methods for producing achromatic retarders, the phase shift, or retardation, in each waveplate is determined by the thickness of the waveplate. During fabrication the thickness of a waveplate made from bulk crystal must be controlled to a fraction of a wavelength over the optical aperture, which is difficult. This difficulty increases the cost of fabrication.
This leads to a second desirable characteristic for retarders to be used for imaging circular polarization: thin planar structure fabricated using microfabrication methods. These microfabrication methods have been developed for the manufacturing of integrated circuits and micro-electromechanical systems (MEMS) and are routinely used to produce thin planar structures with features having dimensions of a fraction of a wavelength.
A thin planar structure which has been studied for use as a waveplate is a surface-relief grating. A surface-relief grating is a series of identical parallel linear ridges arranged on a planar substrate. Waveplates formed from surface-relief gratings are described in C. W. Haggans et al., xe2x80x9cPolarization transformation properties of high spatial frequency surface-relief gratings and their applications,xe2x80x9d in Micro-optics: Elements, systems and applications, H. P. Herzig, Editor (Taylor and Francis, London, 1997). They offer the following advantages over waveplates made of bulk crystals.
Bulk crystals are processed one crystal at a time. Thin planar structures can be fabricated in a batch process to reduce cost. For example, several waveplates may be fabricated simultaneously on a single substrate and several substrates can be processed simultaneously.
Optical devices with larger aperture require larger size bulk crystals. Larger size bulk crystals are more difficult and more costly to fabricate. Using microfabrication methods, waveplates can be made as a continuous thin planar structure over a large area substrate in order to provide a large optical aperture at much lower cost.
Thin planar structures are lightweight compared to bulk crystals. Lightweight is important for an imaging sensor to be portable. Lighter weight also leads to lower cost for the mechanical structure supporting the optical elements.
A waveplates made of a birefringent bulk crystal often has a fast axis that has spatial variation due to nonuniformity in the growth of the crystals. This causes a variation of the phase shift over the aperture. Surface-relief gratings have their fast axes determined, not by the material, but instead by the structure of fabricated ridges. Using microfabrication methods the structures have very high uniformity and the spatial variation of the fast axes is negligible. Retarders made with surface-relief gratings will have negligible variation in phase shift over the aperture.
The handling of bulk crystals restricts the sizes of waveplates to macroscopic dimensions. However, retarders made as thin planar structures can be microscopic in size. Microscopic retarders can be used in optical fiber systems and optical signal processing systems. Microscopic retarders can also be fabricated on a substrate in one- and two-dimensional arrays. Two-dimensional arrays of retarders are of interest for use with two-dimensional photodetector arrays, called focal plane arrays, in imaging sensor systems.
A third desirable characteristic for retarders considered for use in imaging systems is a large field angle, also called acceptance angle. The angle of incidence is the angle between a light ray and the main optical direction of the retarder. For a light ray, along the main optical direction in a quarter-wave retarder, the phase shift is 90 degrees. As the light ray deviates from the main optical direction, the angle of incidence becomes larger, and the phase shift deviates from 90 degrees. The maximum tolerable deviation of phase shift defines the maximum angle of incidence for the retarder. The field angle of the retarder is twice the maximum angle of incidence.
A large maximum field angle is advantageous in an imaging system. In general, the sensitivity of a sensor increases as the aperture of the optical system is increased, because more light enters the sensor. The f-number decreases, and light is incident on the retarder in a wider range of angles, so a larger field angle is required.
Both retarders made of bulk crystals and waveplates made from a single surface-relief grating have maximum field angles too small for practical use in a sensor to image circular polarization.
A fourth desirable characteristic for retarders considered for use in imaging systems is insensitivity to temperature variations. The phase shift introduced by a retarder should not vary with its temperature. This insensitivity in important in portable systems or sensors for use in extreme temperatures such as in cryogenic environments, in hot engines, or in aircraft, missiles, and spacecraft.
The phase shift is determined by the thickness of a waveplate and the velocities of light waves with oscillations along the fast and slow axes. The thickness will change with temperature as characterized by the coefficient of thermal expansion of the waveplate material. The two velocities will also vary with temperature, since the indexes of refraction of materials vary with temperature. Retarders made of bulk crystals and waveplates made from a single surface-relief grating have phase shifts which are temperature dependent.
A fifth desirable characteristic for retarders considered for use in imaging systems is insensitivity to process biases. A process bias is an unintentional change in a dimension which uniformly affects all fabricated devices. For example, if a batch of waveplates were made with a thickness slightly too large, retarders made using a combination of these waveplates should be insensitive to the thickness change and provide a phase shift sufficiently close to the intended value.
An apparatus and method to convert circular polarized light into linearly polarized light over a wide range of wavelengths is provided by utilizing a first surface-relief grating functioning as a quarter-wave waveplate and a second surface-relief grating functioning as a half-wave waveplate. A plurality of such devices are arranged in a two-dimensional array and combined with an array of linear polarizers and an array of photodetectors to form a polarization imaging sensor.
Accordingly several objects and advantages of our invention are as follows.
Object 1. The present invention provides an optical retarder which is achromatic and has thin planar structure.
Object 2. The present invention provides an optical retarder which has a larger field angle than bulk crystal retarders and waveplates made with a single surface-relief grating. Larger field angle is a new and unexpected advantage, not appreciated in the prior art.
Object 3. The present invention provides an optical retarder which is insensitive to variations in temperature. Insensitivity to temperature is a new and unexpected advantage, not appreciated in the prior art.
Object 4. The present invention provides an optical retarder which is insensitive to process biases. Insensitivity to process biases is a new and unexpected advantage, not appreciated in the prior art.
Object 4. The present invention provides an optical retarder which can be fabricated using micromachining and micro-electromechanical processing methods.
Object 5. The present invention provides an optical retarder which can be fabricated in a batch process as several devices on a substrate or as several substrates processed simultaneously.
Object 6. The present invention provides a optical retarder which can be formed as continuous structure over a large area substrate in order to provide a large optical aperture.
Object 7. The present invention provides an optical retarder with negligible variation in phase shift over the aperture.
Object 8. The present invention provides an achromatic optical retarder which can be fabricated in microscopic dimensions for use with other microscale optical devices such as in optical fiber systems, in optical signal processing systems, and in imaging systems using focal plane arrays. The ability to fabricate the achromatic retarder in microscale dimensions is a new and unexpected advantage, not appreciated in the prior art.
Object 9. The present invention provides an achromatic optical retarder which can be fabricated on a substrate in one- and two-dimensional arrays. Two-dimensional arrays of retarders are of interest for use with two-dimensional photodetector arrays, called focal plane arrays, in imaging sensor systems. The ability to fabricate the achromatic polarizer in one- and two-dimensional arrays is a new and unexpected advantage, not appreciated in the prior art.
Object 10. The present invention provides an achromatic circular-to-linear polarizer which can be fabricated on a substrate in a two-dimensional array and combined with an array of linear polarizers and an array of photodetectors to form a polarization imaging sensor. The ability to form an sensor to image circular polarization is a new and unexpected advantage, not appreciated in the prior art.
Object 11. The present invention provides a sensor to image circular polarization without the mechanical rotation of a linear polarizer or a waveplate. The ability to form a sensor to image circular polarization without mechanical rotation is a new and unexpected advantage, not appreciated in the prior art.
Object 12. The present invention provides an achromatic optical retarder which can be fabricated on a substrate in a two-dimensional array and combined with an array of linear polarizers on the same substrate, where the substrate will maintain the alignment of each retarder relative to its corresponding linear polarizer.
Further objects and advantages of the invention will become apparent from a consideration of the drawings and ensuing description.